During the processing of hydrocarbon feedstocks in, e.g., petrochemical refineries, sulfur containing compounds are removed from the feedstock by contact with a caustic (i.e., alkaline) solution, into which the sulfur containing compounds (e.g., thiosulfates) dissolve. Among the materials which can be treated this way are natural gas, fuel gas, liquefied petroleum gas, pentane mixtures, light straight run naphtha, light thermally cracked naphtha, full straight run naphtha, full FCC cracked naphtha, heavy SR naphtha, aviation turbine fuel, kerosene, and distillate fuels with boiling points up to 350° C. (The skilled artisan will recognize that this listing is exemplary and hardly cumulative or exhaustive). While this is a standard, efficient process, the result is a spent, caustic solution which must be addressed. These spent solutions, containing dissolved sulfur compounds, must be treated to avoid environmental and other problems.
Conventional techniques are available for dealing with the problem of the spent caustic solutions. Most common is so-called “wet air oxidation,” where after oxidation with air, disulfides result, and the caustic solution is regenerated. The mixed disulfides, sometimes referred to as “disulfide oil,” (DSO) can then be treated via, e.g., hydrotreating or hydrocracking. See, e.g., Published U.S. Patent Application 2016/0108333, incorporated by reference. While this is a useful process, it requires hydrogen, which makes it expensive. Burning, or otherwise disposing of the disulphide oil is possible, but far from desirable.
The field of petrochemistry is familiar with the so-called “MEROX” process. As its name suggests, the MEROX process involves oxidation of mercaptans in hydrocarbon mixtures, via the basic reaction:RSH+¼O2→½RSSR+½H2O.
The process requires an organometallic catalyst and an alkaline solution in order to accelerate oxidation and to proceed at an economically practical rate. The process is important for the disposition of sulfur containing compounds removed from hydrocarbon feedstocks in petroleum refining.
In the equation provided supra, “R” is a hydrocarbon chain of variable length, which may be saturated, unsaturated, branched, cyclic, or any form of hydrocarbon found in petroleum feedstocks, crude oil, etc. Generally, these feedstocks contain mixtures of compounds of the formula RSH, where R and R′, infra, can contain 1 to 10 carbon atoms, or even more then 10, but R and R′, infra, preferably contains 1-8 carbons.
Indeed, the wide variety of hydrocarbons makes the following reaction scheme more accurate.2R′SH+2RSH+O2→2R′SSR+2H2O
MEROX processes involve either liquid streams, or mixtures of liquid and gas streams.
When the starting material, i.e., the feedstock, is liquid only, the disulphides remain in the reaction product and the total sulfur content does not change. The vapor pressure of the resulting disulphides is low as compared to mercaptans, so their presence is far less objectionable then that of mercaptans. On the other hand, the disulphides are not environmentally acceptable and their disposal is difficult.
In practice, liquid stream feedstocks are usually treated in a fixed bed reactor system over a catalyst, such as activated charcoal impregnated with a MEROX reagent, and wetted by a caustic, alkaline solution. Air is injected into the feedstock ahead of the reactor, and as the feedstock passes through the wetted, catalyst impregnated bed, mercaptans are oxidized to disulphides. Because the disulphides are insoluble in the caustic, alkaline solution, they remain with the other hydrocarbons, and complex processes to remove them must be employed.
When the feedstock is a mix of gas and liquid, the disulphides can be extracted into the alkaline solution. The degree of extraction depends, inter alia, on the molecular weight of the mercaptans, their degree of branching, the concentration of the alkaline solution, and the reaction temperatures.
The disulphide oil which results from the MEROX reaction discussed supra is a mix of various disulphides. Table 1, which follows, shows the disulphide oil composition obtained following oxidation of compositions containing propane, butane, and mercaptans:
TABLE 1Composition of disulfide oil.Disulfide OilW %BPMWSulfur, W %Di-Methyl Di-Sulfide181109468.1Di-Ethyl Di-Sulfide4815212252.5Di-Propyl Di-Sulfide3319515042.7Methyl Ethyl Di-Sulfide112110859.3Ethyl Propyl Di-Sulfide216813647.1Total100158.64126.352.0
As noted, supra, the disulphide oil is problematic. It can be added to fuel oil, or be further processed in a hydrotreating/hydrocracking unit, which increases expenses because of the need for hydrogen. Given the issues surrounding this byproduct of hydrocarbon processing, there is a need to utilize the byproduct and/or dispose of it, within the confines of a refinery.
Hydroprocessing catalysts, especially those containing Ni and/or Mo oxides, as well as Co and/or W oxides, require activation to achieve maximum potential conversion of the oxides to corresponding sulfides, is the industry standard, viz:
TABLE 2Activating reactions.Metal OxideReactantskProductsMoO31MoO3 + H2 + 2H2S−>MoS2 + 3H2ONiO3NiO + H2 + 2H2S−>Ni3S2 + 3H2OCoO9CoO + H2 + 8H2S −>CO9S8 + 9H2OWO31WO3 + H2 + 2H2S−>WS2 + 3H2O
Standard methods to carry out this reaction include in-situ gas phase activation, or liquid phase activation, each of which may use a sulfur spiking agent, but need not do so. An ex situ pre-activated catalyst can also be used. Of these options, liquid phase in-situ activation with a sulfur spiking agent is most common. The following table lists the most frequently used agents, and some of their properties:
TABLE 3Catalyst activating agents.Sulfur,MW,NameW %g/g-molSGBP, °C.Di-methyl-di-sulfide (DMDS)68941.060109di-methyl-sulfide (QMS)52620.84036di-methyl-sulfoxide (DMSO)41781.100189N-butyl-Mercaptan36900.85096Tertiary-butyl-poly-sulfide (TBPS)542421.100—Tertiary-nonyl-poly-sulfide (TNPS)374141.040—
The materials of Table 3 are converted fully to H2S and hydrocarbons, while the H2S converts the oxides into sulfides, as shown by Table 2.
The conditions under which disulphides, mercaptans, and mixtures of these decompose to hydrocarbons and hydrogen sulphide include a hydrotreating catalyst, and temperatures from 150-260° C.
As noted, supra, the prior art has long used commercially available disulphides to activate hydroprocessing and hydrocracking catalysts. This invention unites two separate areas of technology used, in petrochemical processes. In brief, it involves removal of sulfur compounds by caustic solvents and their conversion to mercaptans and/or disulphides and then use of this potentially problematic waste product to activate catalysts directly.
How this is accomplished will be seen in the disclosure which follows.